1. Field of Invention
The present invention relates to an exhaust line used for driving out exhaust gas and sewage in a semiconductor structure. More particularly, the present invention relates to an exhaust line of a chemical-mechanical polisher.
2. Description of Related Art
With progress of a semiconductor technology, a spin-on glass (SOG) method that is a conventional method for planarization is not sufficient for the progressing semiconductor technology. Chemical-mechanical polishing (CMP) is a new technology that is gradually replacing SOG. CMP had become a popular method for planarization. CMP uses "polishing pad", which is similar to an abrasive paper, to polish a non-uniform wafer surface. During the polishing process, a suitable chemical reagent is added to help the polishing process. As long as the polishing process is well controlled, CMP can provide global planarization and a high degreed of planarization for wafers.
In the polishing process, a chemical reagent, which is called "slurry", is added to a wafer. Slurry is mainly composed of colloidal silica, or a mixture of dispersed alumina, alkaline potassium hydroxide (KOH), and ammonium hydroxide (NH.sub.4 OH). The slurry reacts with the material to be removed, and breaks the bonds between molecules. Then, the material reacting with the slurry is removed by mechanical force applied by the polishing pad. The main parameters controlling a CMP process include the slurry, the polishing pad material, the operating temperature, and the pH of the slurry.
In general, when metallic material is polished, hydrogen peroxide (H.sub.2 O.sub.2) solution is added into slurry to control the pH. When oxide material is polished, potassium hydroxide (KOH) solution is added into the slurry to control the pH. Since H.sub.2 O.sub.2 and NH.sub.4 OH vapors are hazardous to human health, exhaust lines are necessarily designed on a chemical-mechanical polisher.
Referring to FIG. 1, FIG. 1 is a schematic, structural view of a conventional exhaust line of a chemical-mechanical polisher. A polishing chamber 100 is a closed system. A polishing table 11 is centrally located in the polishing chamber 100. A plurality of polishing pads 10 is on the polishing table 11. A gas outlet 12, connecting with a gas exhaust line, is at the top of the polishing chamber 100. A liquid outlet 14, connecting with a liquid exhaust line, is on the polishing table 11. While performing a polishing process, a wafer is first placed on the polishing pad 10. Using a polishing pad and adding slurry, the wafer is polished. During the polishing process, exhaust gases and sewage generate in the polishing chamber 100 due to reaction of the slurry and the material to be removed from the wafer. The exhaust gas is driven out through the gas outlet 12, using a pump. The sewage is driven out through the liquid outlet 14.
However, slurry is a suspension, and some solids are suspended in the slurry. If solvent (H.sub.2 O) of the slurry evaporates, solids remain. During the polishing process, the exhaust gas is driven out through the gas outlet 12 and the sewage is simultaneously driven out through the liquid outlet 14, so that a portion of the slurry evaporates. Some solids remained on the walls of the gas exhaust line. When the polisher is used for a long time, aggregated grains of solids are generated on the walls of the gas exhaust line. Due to gravity, the grains may fall on the polishing pad 10. While performing the polishing process, scratches, caused by contaminating grains on the wafer, are easily generated on the wafer, so that device yield is thus reduced.